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Introduction into Hd Audio

Bitstream

Bitstream audio refers to a succession of 1's and 0's transmitted from a
player typically to a receiver or A/V preamp. The receiver then decodes
this information and amplifies the resulting audio signal to drive your
speakers. This information can be transmitted either through SPDIF
(optical or digital coax) or HDMI connections depending on the
compression method.

PCM or Pulse Code Modulation "is a digital representation of an analog
signal where the magnitude of the signal is sampled regularly at uniform
intervals, then quantized to a series of symbols in a digital (usually
binary) code." (From the PCM WIkipedia article.) There are compressed
forms of PCM audio but the PCM soundtracks that you see in BluRay discs
are uncompressed.

Linear PCM or LPCM can be included on the disc itself or the player can
derive LPCM from other digital tracks and transport this through the
player's digital connections.

Is there an audible difference?

Given the right equipment and the right conditions a compressed lossless
Dolby True HD soundtrack should sound the same as an uncompressed LPCM
soundtrack.

See this comparison between the two on "The Departed." The LPCM
soundtrack is on the BluRay disc and the Dolby True HD trak is on the HD
DVD disc:

"Now, how do the PCM and TrueHD tracks compare? Given this historic
opportunity, I decided to conduct a little experiment. I invited a
friend over, who is a big movie and music buff, but not particularly
technical. He knows good audio when he hears it, yet doesn't know a PCM
from an RPM from R.E.M. In other words, he's Joe Six-Pack with a great
ear. Anyway, together we conducted a "blind" audio test -- we select ten
short sequences from the film, and listened to a compare of each. We
took turns firing up each scene, and selecting which one sounded better,
with no knowledge of which sample was the Blu-ray and which the HD DVD.

After writing down our answers on little scraps of paper (note that we
didn't throw them into a hat -- we aren't that dorky), the results were
interesting. Out of the twenty comparisons (ten for him, ten for me), we
could only detect differences on four scenes total. But of those four,
we both always preferred the PCM track, if only a smidgeon. For example,
there is a scene in involving an attempted trade bust between the
Costello character and a Chinese gang. There is a sound of a gun firing
that we went back over a few times, and as silly as it sounds, the force
and impact of the sounds was a shade more realistic in PCM. Also a
beneficiary of the uncompressed mix is the music, as this is a film
brimming with rock songs. The first scene we picked featured the Rolling
Stone's "Gimme Shelter," and again the PCM track boasted a slightly
more spacious feel to the music in all channels -- as if the very
highest end of the frequency range was more palpable.

Granted, these are very slight differences and subjective preferences.
Had we not blindfolded each other (figuratively speaking, of course) and
been flipping back and forth between discs like one of those old
Coke-Pepsi commercials, such deviations likely would have been
imperceptible. It is also certain that the average listener wouldn't be
able to tell the difference without possessing the ears of a dog. Still,
in this case I give a slight edge to the PCM track, though a comparison
between a single title hardly qualifies as the final word. If nothing
else, it made me realize that if all the studios dumped this dueling
audio format business and went all-PCM, I can't say I would be likely to
complain... "

As long as the Dolby True HD soundtrack is decoded properly, the
resulting LPCM signal should be exactly the same as the uncompressed
LPCM track or the studio master.